1. Field of the Invention
The present invention relates generally to anodization of a pellet of a valve metal powder for use as an anode in an electrolytic or an electrochemical/electrolytic hybrid capacitor. Anodization of the valve metal pellet or body is performed in an electrolyte that is flowed through the valve metal pellet to produce a stable oxide layer with a high dielectric constant. A preferred valve metal is tantalum.
2. Description of Related Art
In general, electrolytic capacitors comprise anodes and cathodes that are physically segregated from each other by a porous separator material impregnated with an ionically conductive working electrolyte. The working electrolyte is typically composed of water, solvent(s), and salt(s) of weak inorganic and/or organic acids. The anodes are of a valve metal having its exposed pore surface coated with a film of the corresponding oxide serving as a dielectric. Valve metals include, but are not limited to, aluminum, tantalum, niobium, titanium, zirconium, hafnium, and alloys thereof. The valve metal can be in any conventional form including etched foil, sintered powders, or other porous structures. Anodizing the valve metals in an appropriate anodizing electrolyte forms a dielectric oxide film thereon. The film thickness increases with the anodizing voltage. The desired oxide film thickness is determined by the capacitor working voltage, operation temperature and other performance requirements.
It is believed that locally excessive temperatures and insufficient material transport in porous valve metal bodies during anodizing (especially for anodization of high voltage, relatively large, pressed and sintered tantalum powder pellets) causes breakdown or poor anode electrical properties. There have been numerous attempts to solve these problems by improving the heat and electrolyte transport between the pellets and the bulk electrolytes. Some of the prior art methods include: controlling the anodizing current density; mechanical, sonic, or ultrasonic agitation of the electrolyte; anodizing by combining control of voltage/current and controlled rest steps (U.S. Pat. No. 6,231,993 to Stephenson et al.); and controlled pulses of the voltage/current (U.S. Pat. No. 6,802,951 to Hossick-Schott). These methods require sophisticated electronics for current/voltage/power control and frequent on/off switches that increase anodizing time. Additionally, it is believed that the eruptive increase in current/voltage in the case of pulsed anodizing may cause early breakdown and poor oxide quality.
U.S. Patent Application Pub. No. 2006/0196774 to Liu et al. discloses a method of anodizing valve metals by self-adjusted current and power. U.S. Patent Application Pub. No. 2006/0191796 to Muffoletto et al, discloses a method of anodizing valve metals by controlled power. The disclosed methods provide improved anodization of valve metals by application of electrical power to the valve metal at specific levels and in specifically timed on-off sequences. These publications are assigned to the assignee of the present invention and incorporated herein by reference
Thus, it is known that the conditions for anodizing must be carefully controlled in order to provide a suitable oxide layer. In particular, the current applied during formation is generally kept low to avoid electrical breakdown. Additionally, the current may be turned off or reduced for periods of minutes to several hours during formation, as described in the above patent applications of Liu et al. and Muffoletto et al. The low current and the “off” periods make the formation process very slow; thirty hours or more may be required to form an anode pellet for a capacitor. Although there is no general agreement on the precise mechanism by which formation current leads to electrical breakdown, it is thought by some that the current must be kept low in order to avoid overheating the pellet. Others believe that the formation process is limited by the relatively slow diffusion of electrolyte components or products of the electrochemical oxidation.
In any event, the speed of the formation process is limited by the ability of conventional means to remove heat and/or “used” electrolyte from the interior of the pellet. “Conventional” in this sense means relying on stirring by means of pumps or stirrers to provide sufficient convective flow of electrolyte around an anode pellet immersed in an electrolyte bath. To the extent that there is any increased convective heat and/or mass transfer in the electrolyte at the anode pellet, the enhancement likely only occurs at the outer (visible) surface of the pellet, and not at the internal pore surfaces.
Consequently, it has been found that there are greater difficulties in preparing capacitor anode pellets for high voltage use, and these difficulties are increased as the desired thickness of the oxide layer on the pellet surface increases. Among these difficulties are electrical breakdown which occurs with increasing likelihood at higher formation voltages, and so-called “gray-out”. “Gray-out” refers to the appearance of gray or whitish oxide patches on the pellet due to the formation of crystalline oxide, which is more “electrically leaky”, i.e., having localized areas of lower electrical resistance, than the desired amorphous oxide. This may occur when formation is carried out using a low current density with the intention of reducing electrical breakdowns.
Therefore, there remains a need for an apparatus and a method for manufacturing a valve metal anode such as of the kind typically used in an electrolytic capacitor. Particularly, it is desirable to provide valve metal anodes with dielectric coatings having improved oxide quality and higher breakdown voltages. In that light, the present invention teaches an apparatus and a anodization method that reduces process time and provides a better quality dielectric oxide. With the present invention, the current density used during formation of the oxide layer may be increased and the time required for oxide layer formation decreased. These attributes substantially improve the economics of manufacturing valve metal anodes.
Therefore, although this invention is, in principle, applicable to all valve metal anodes, it is particularly useful for anodizing a high voltage sintered tantalum pellet for use in an electrolytic capacitor.